The present invention relates to a method for forming inorganic membranes, more particularly it relates to a method for forming zeolite membranes.
It is known to form zeolite membranes by crystallisation of zeolites from gel solution onto a substrate such as a metal mesh substrate or a porous ceramic substrate. The membranes which can be used in the present invention can be formed by any method, for example by crystallisation from a gel or solution, by plasma deposition or by any other method such as electro-deposition of crystals on conducting substrates e.g. as described in DE 4109037.
When the membrane comprising a film of zeo-type material is prepared by crystallisation from a synthesis gel, any of the methods described in the prior art can be used for example, as described in EP-A-57049. EP-A-104800. EP-A-2899 and EP-A-2900. Standard text books by D W Breck (xe2x80x9cZeolites Molecular Sieves, Structure Chemistry and Usexe2x80x9d) published by John Wiley (1974) and P. A Jacobs and J. A Martens (Studies in Surface Science and Catalysis No. 33, Synthesis of High Silica Alumino silicate Zeolitesxe2x80x9d published by Elsevier (1987), describe many such synthesis gels. The process which can be used includes conventional syntheses of zeo-type materials, except that the synthesis is carried out in the presence of the porous support. Most commonly, gels are crystallised by the application of heat.
Preferred methods are disclosed in our copending patent applications PCT/GB96/00243, PCT/GB97/00928 and PCT/GB 97/00635.
It is known to pre-treat the substrate before deposition of the zeolite onto the mesh e.g. as described in patent application U.S. Pat. No. 5,716,527.
We have now discovered a method of pre-treating a porous substrate to give an improved membrane.
According to the invention there is provided a method of treating a porous substrate which method comprises passing a suspension of zeolite particles in a liquid suspension though the porous support until a coating of zeolite particles is formed on the support surface and in which the particles have a diameter of between 20 xcexcm and 0.1 xcexcm.
We have found that a narrow size range of zeolite particles gives unexpectedly superior results.
Preferably the range of particles is achieved by mixing together particles of different size distribution. Zeolite particles, when prepared will have a particle size distribution, if these particles are ground or pulverised the average size is reduced and the distribution of sizes changed. If these particles are mixed with the unground particles a mixture can be obtained with a preferred size distribution for use in the present invention.
Preferably the zeolite particles are a mixture of particles which conforms approximately to a modified Fuller curve.
Fuller curves are grading curves which give the minimum void space and closest packing for sands and other mineral aggregates containing particles of varying sizes. The shape of the Fuller curve will depend on the maximum particle size, but will be a single curve for any given maximum particle size.
The Fuller curves are described in a paper by Fuller and Thomson entitled xe2x80x9cThe laws of Proportioning Concretexe2x80x9d published in the Transactions of the American Society of Civil Engineers, 1907, 59, pages 67-172. The Fuller curves are identified by their maximum particle size, e.g. in the present invention a preferred particle distribution is a 20 xcexcm Fuller curve.
The porous supports on which zeo-type membranes are formed and which can be used in the present invention include those formed of metals, ceramics, glass, mineral, carbon or polymer fibres or cellulosic or organic or inorganic polymers. Suitable metals include titanium, chromium and alloys such as those sold under the Trade Marks xe2x80x9cFecralloyxe2x80x9d and xe2x80x9cHastalloyxe2x80x9d and stainless steels. The porous supports may be formed of a mesh or from sintered metal particles or a mixture of both. These are commonly sold in the form of filters.
Porous ceramics such as glass mineral or carbon materials are preferred including alumina, porous silicon and other carbides, clays and other silicates and porous silica. If desired, the support can be a zeolite formed by compression or using a binder. The shape of the support is not critical, for example, flat sheet, tubular, wound spiral, etc. can be used.
The porous support can be also be a granular solid e.g. formed of particles of a closely packed material such as a pellitised catalyst.
The present invention can be used with porous supports of any suitable size although, for large flux rates through a membrane, large pore sizes are preferred. Preferably pore sizes of 0.01 to 2,000 microns, more preferably of 0.1 to 200 and ideally of 0.1 to 20 microns are used. Pore sizes up to 300 microns can be determined by bubble point pressure as specified in ISO 4003. Larger pore sizes can be measured by microscopic methods.
The method of the present invention is particularly useful for forming a zeolite membrane on the inside of a tubular porous support e.g. in a ceramic monolith.
In this application of the method of the invention a suspension of the zeolite particles is passed through the bore of the tube and out through the walls of the tube so as to deposit a layer of zeolite membranes on the inner surface of the tube.
The process is continued until the desired thickness of particles are deposited on the inner surface or a pre-determined flux of the liquid passes through the tube walls.
The suspension of zeolite particles is preferably an aqueous suspension of the particles and the suspension can be formed by forming a mixture of zeolite particles and water and grinding this mixture in a grinder until the particles are of the requisite size for use in the invention. The particles can be ground or milled in a conventional grinder or mill. Alternatively the particles can be reduced in size by a combination of shear, cavitation and impact e.g. as occurs in a microniser. These ground particles can be mixed with particles of a larger size so that the required particle size distribution is obtained.
After treatment a zeolite membrane can be formed on the treated substrate by any of the methods described in the prior art referred to above and the method is particularly useful when the membrane is formed by gel crystallisation.